This invention relates to an optically-compensated zoom lens with a zoom portion comprised of a positive first, a negative second and a positive third lens group of which the first and third are movable along a common optical axis in fixedly spaced relation to each other, while the second lens group remains stationary during zooming.
The zoom lenses are generally classified into two categories by the shift of focal plane compensation technique, i.e., mechanically-compensated zoom lens and optically-compensated zoom lens. Of these the latter gives rise to a difficult problem when designed to be used as a zoom lens for a still camera. This problem is solved by decreasing the length of translation of the movable portion, i.e. the first lens group and the third lens group in such a manner that, while the relative aperture is maintained at a somewhat large level, high grade imagery is preserved throughout the zooming range by limiting the image shift within the depth of focus.
A practical example of the optically-compensated zoom lens adapted for use in a still camera is disclosed in Japanese Patent Application No. Sho 49-23911, as comprising four lens groups of which the first, third and fourth are of positive power, and of which the second is of negative power. And the second and fourth lens groups are maintained stationary during zooming, and the first and third lens groups are movable for zooming in fixedly spaced relation to each other on the opposite side of the second lens group. And each of the first and third lens groups is provided with a divergent contact surface of the positive sign. In the zoom system, each lens group of the zoom portion generally needs to be separately achromatized for good correction of the aberrations, and for that purpose, each lens group needs to be provided with a contact surface, with the arrangement of the positive, negative and positive lens groups in the zooming portion of the optically-compensated zoom lens, however, when the divergent contact surfaces in the first and third lens groups are both positive in curvature, the distortion tends to be increased in pincushion sense at the telephoto side, when they are both negative, over-correction of astigmatism will result.
Another problem which arises when zooming is to restrict a shift of focal plane to a fraction of the depth of focus. In the case of the aforesaid type optically-compensated zoom lens, the magnitude of shift of the back focus, that is, the image plane varies with zooming, following a cubic curve of the movement of the movable portion. Letting S denotes the amount of movement of the movable portion measured from the telephoto position to the wide angle position, the magnitude of the shift of focal plane becomes maximum at two points of zooming which are generally near 1/4S and 3/4S. On the other hand, if the best focus for an objective lens in a 35 m/m still camera is assumed to be determined by the circle of least confusion at 0.033 mm, the depth of focus is expressed as 0.033.times.F-number. Hence, for good image quality, the difference between the back focus when in the telephoto setting and any of those when in the wide angle setting and intermediate settings for 1/4S and 3/4S must be smaller than the depth of focus expressed as 0.033.times. F-number.
As the size of aperture opening is increased, the depth of focus is decreased. Further an increase in the zoom ratio without involving any great increase the bulk and size of the complete objective, in other words, the length of translation of the movable portion calls for an increase in the refractive powers of the each lens group in the zooming portion, which will result in the tendency of increasing the image shift. Accordingly, in designing an optically-compensated zoom lens objective, it has been difficult to achieve simultaneous minimization of the bulk and size of the complete objective and the image shift for high grade imaging performance throughout the entire zooming range.